1. Technical Field
The present invention relates to gravel pack logging and, more particularly, to an improved method for providing a reliable quantitative evaluation of gravel pack quality.
2. Prior Art
Many oil wells are completed with internal gravel pack assemblies to prevent sand infiltration into the production tubing. A reliable method of evaluating the quality of the gravel pack in situ, at the time of its construction, is needed in order to improve the effectiveness of the gravel pack and to reduce the chances that a workover will be required later.
Prior investigations of this problem have indicated the applicability of wireline logging techniques to the evaluation of gravel packs. In a paper entitled "Gravel Pack Evaluation", first presented (Paper SPE 11232) at the 57th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, New Orleans, LA, September, 1982, published Journal of Petroleum Technology, September, 1983, pp. 1611-1616, M.R. Neal described the responses of three well logging tools, the compensated neutron tool (neutron source and two neutron detectors), the nuclear fluid density meter tool (gamma ray source and one gamma ray detector), and the dual-spacing gamma ray tool (gamma ray source and two gamma ray detectors), to various gravel pack situations and showed that each tool responded well to changes in density of the material in the annulus between the screen (of the gravel pack hardware) and the casing. Although Neal's work provided useful qualitative information concerning gravel pack quality, it did not provide a procedure by which a quantitative evaluation could be made.
Further research by M.R. Neal and J.F. Carroll, as reported in a paper entitled "A Quantitative Approach to Gravel Pack Evaluation", 6th SPE of AIME Formation Damage Symposium, Bakersfield, CA, Feb. 13-14, 1984, demonstrated that tool response (count rate) could be directly related to the percent void space in the gravel pack, and led to the development of interpretive procedures for determining percent packing when field hardware is the same as that used for laboratory calibration measurements and for making a quick-look quantitative approximation when the well hardware differs from laboratory hardware.
Furthermore, it is known to use for investigating a gravel pack, a tool comprising a gamma ray source and a gamma ray detector, such as described in U.S. Pat. No. 4,587,423 issued to James R. Boyce on May 6, 1986.
Although such tools are satisfactory, they show some limitations.
Firstly, such known tools use a chemical nuclear radioactive source; any sticking of the tool in the well may have potential serious consequences on the environment, not to speak about the pecuniary point of view.
Secondly, in highly deviated wells and horizontal wells, gravel pack separates from completion fluid, owing to their respective different densities. Gravel pack material settles on the low side of the borehole, leaving the upper section of the borehole improperly packed. In view of obviating this natural trend, such wells are gravel packed with gravel pack and completion fluid having equal or similar density values. However, this entails that the gravel pack cannot be distinguished from completion fluid, since said known tools carry out density measurements; this prevents any effective measurements of gravel pack quality.
Thirdly, it is a constant preoccupation of one skilled in the art to decrease to the utmost the influence of the formation on the gravel pack measurements.
Fourthly, there is a general trend in the logging techniques to know better and better the gravel pack quality; in other words quantitative results are requested; e.g. in some occasion, the need occurs to locate accurately a pack region which has been partially emptied (following faults in the screen) and refilled with formation material.